The present invention relates to an echo canceller and, more particularly, to an echo canceller for two-wire full duplex digital data transmission system.
Generally, a data transmission system is either two-wire or four-wire system, where "two" and "four" stand for the numbers of physical transmission media directly connected to data circuit terminating equipment (DCE). In a two-wire transmission system, the bidirectional data transmission is performed by a pair of transmission lines having no directivity. On the other hand, in a four-wire transmission system, it is performed by two independent pairs of unidirectional transmission lines. Moreover, the data transmission system has two types of operational modes, that is, full duplex and half-duplex transmission modes. The full duplex transmission means simultaneous transmission in both directions by means of same transmission path, while the half duplex transmission means the alternate transmission in both directions by time-divided use of the same transmission path. It is well known that in order to install a long-distance data transmission system, the two-wire full duplex is superior than four-wire full duplex and two-wire half duplex modes in view of installation cost and transmission efficiency. By two-wire full duplex data transmission system, the speech, data and facsimile communication services have been provided in the forms of analogue signal. Recently, these services are provided also in the forms of digital signal.
On the other hand, the evolution of digital technique promotes the digitalization of telephone exchanger and transmission system. Consequently, an integrated services digital network (ISDN) is developed, by which various communication services, such as speech, data and facsimile communications, are provided by a single digital network integrated by digital facilities. By means of ISDN system, the economization of facilities, such as telephone exchanger and transmission path, can be attained. Moreover, the transmission quality can be improved, because the direct digital terminal-to-terminal transmission is available.
In order to receive the various services from ISDN, it is preferable from the consideration of the efficient use of transmission path that the user can access the ISDN through two-wire full duplex digital transmission path. However, the two-wire full duplex digital data transmission requires an echo canceller, by which echoes generated in the transmission path are suppressed by digital signal processing technique.
A conventional echo canceller suitable for two-wire full duplex digital data transmission system comprises an adaptive digital filter composed of a transversal filter. In the echo canceller of the kind, an echo replica is generated by estimating adaptively the time-varying transmission characteristics of echo path by means of adaptive filter, and then the echo component is cancelled by subtracting echo replica from a received signal containing echo signal generated in echo path. Generally, the estimation of tap coefficients of the adaptive filter employs the mean square error algorithm, which is expected to relatively stably converge.
An example of the echo canceller of the kind is described in U.S. Pat. No. 4,087,654. However, the conventional echo canceller has the following difficulty. Namely, the impulse response of echo canceller is not finite, but lasts indefinitely with exponential decay, the time constant of which is determined by the impedance of a hybrid transformer and the matching impedance constituting a hybrid circuit. The residual echo signal at echo tail portion not cancelled by echo canceller contains the low frequency component of several tens Herz, which becomes low frequency noise to degrade the communication quality. In order to assure the required suppression of residual echo by cancelling the low frequency component existing during long time interval, it is usually necessary to select the number of taps of adaptive filter to be 64 or more. In detail, the residual echo r.sub.n is given by eq. (1), ##EQU1## where N denotes the number of taps of adaptive filter, {a.sub.n } denotes the transmitted symbol, while A and .alpha. denote amplitude and decaying rate of echo tail, respectively. The noise power N.sub.PE due to residual signal r.sub.n is given by eq. (2), ##EQU2## In a practical system, the average transmission power of 2B1Q (2 binary 1 quaternary) symbol is E[{a.sub.n.sup.2 }]=5 V, assuming that the transmission signal level for symbol +1 is 1 volt, while amplitude and decaying rate of echo tail are A.sup.2 =3.7 .times.10.sup.-6 and .alpha.=0.969, respectively. The received signal power S.sub.P =0.0005, if the loss in the transmission path is considered. In order to guarantee S/N ratio of 20 dB, the noise power level should be N.sub.PE &lt;S.sub.P /100=0.000005. Thus, the eq. (2) gives the number of taps N&lt;64.
Consequently, as the number of taps of adaptive filter increases, the hardware structure and the computation indispensably increase.